H. M. Lo

Highly reliable nitride-based LEDs with SPS+ITO upper contacts

Nitride-based blue light emitting diodes (LEDs) with an n/sup +/-short period superlattice (SPS) tunnel contact layer and an indium tin oxide (ITO) transparent contact were fabricated. Compared with conventional nitride-based LEDs with Ni/Au upper contacts, it was found that we could achieve a 60% increase in electroluminescence (EL) intensity by using ITO upper contacts. However, it was also found that the lifetime of ITO LEDs were much shorter. Furthermore, it was found that we could achieve a longer lifetime and a smaller reverse leakage current (I/sub R/) by the deposition of a SiO/sub 2/ layer on top of the ITO LEDs.

Nitride-based flip-chip ITO LEDs

Nitride-based flip-chip indium-tin-oxide (ITO) light-emitting diodes (LEDs) were successfully fabricated. It was found that the forward voltage and the 20 mA output power of the flip-chip ITO LED were 3.32 V and 14.5 mW, respectively. Although the operation voltage of such a flip-chip ITO LED was slightly larger, it was found that its output power was much larger than those of conventional nonflip-chip LEDs. It was also found that flip-chip ITO LEDs were more reliable.

Nitride-based LEDs with an SPS tunneling contact Layer and an ITO transparent contact

The indium-tin-oxide [ITO(80 nm)] and Ni(5 nm)-Au(10 nm) films were separately deposited on glass substrates, p-GaN layers, n/sup +/-InGaN-GaN short-period-superlattice (SPS) structures, and nitride-based light-emitting diodes (LEDs). It was found that ITO on n/sup +/-SPS structure could provide us an extremely high transparency (i.e., 93.2% at 465 nm) and also a reasonably small specific contact resistance of 1.6/spl times/10/sup -3//spl Omega//spl middot/cm/sup 2/. Although the forward voltage which corresponds to 20-mA operating current for LED with ITO on n/sup +/-SPS upper contact was slightly higher than that of the LED with Ni-Au on n/sup +/-SPS upper contact, a 30% higher output intensity could still be achieved by using ITO on n/sup +/-SPS upper contact. Moreover, the output power of packaged LED with ITO was about twice as large as that of the other conventional Ni-Au LEDs.

Nitride-based LEDs with textured side walls

Nitride-based light-emitting diodes (LEDs) with textured side walls were fabricated. By using plasma-enhanced chemical vapor deposition SiO/sub 2/ layer as the etching mask, we successfully etched the nitride epitaxial layers to achieve wavelike side walls. It was found that such wavelike side walls could mainly enhance the light output at the horizontal directions. With a 20-mA current injection, it was found that the output powers of the LED with textured side walls and normal LED were 9.3 and 8.4 mW, respectively. Furthermore, it was found that such textured side walls will not result in a higher operation voltage.

Nitride based Power Chip with Indium-Tin-Oxide p-Contact and Al Back-Side Reflector

Nitride-based large size (i.e. 1 mm×1 mm) indium-tin-oxide (ITO) light emitting diodes (LEDs) were successfully fabricated. In order to enhance the output intensity of power chips, Al reflector was deposited by e-beam evaporator on the chip backside. It was found that the 350 mA output power was 84.8 mW (W-P-E=7.2%) at 460 nm for the power chip with ITO as p-contacts and Al as back-side reflector. It was also found that ITO power chip with Al reflector was more reliable.

AlGaInP-Based LEDs With a

In this paper, indium-tin-oxide (ITO) films were deposited on p-type GaP films with a AuBe-diffused metal layer to form ohmic contacts. Without the AuBe diffused into p-GaP films, the ITO deposited on p-GaP showed a non-ohmic characteristic. After the AuBe diffused, the ITO deposited on p-GaP displayed a linear current-voltage characteristic and the specific contact resistance showed 2.63 × 10-4 ω-cm2. Furthermore, the specific contact resistance could be improved to 1.57 × 10-4 ω-cm2 when the sample post-ITO-deposition annealed at 400°C. The transmittance of ITO film almost was kept at 90% in the wavelength range of 400-700 nm after thermal annealing. These results revealed that the ITO films can be a suitable transparent current spreading layer for the fabrication of AlGalnP-based light-emitting diodes with an AuBe-diffused metal layer. It was also found that the 20 mA forward voltages measured from LEDs with Device A, Device B, Device C and Device D were 1.97, 1.96, 1.95 and 2.66 V and the light output powers were 4.2, 5.7, 6.0 and 6.3 mW, respectively.

{\rm p}^{+}

Ni (5 nm)/Au (5 nm) and Ni (5 nm)/indium-tin-oxide (ITO) (60 nm) films were deposited onto glass substrates, p-GaN epitaxial layers and nitride-based light-emitting diode (LED) structures. It was found that the normalized transmittance of subjected to rapid thermal annealing at 300°C Ni/ITO film (300°C-RTA) could reach 90.1% at 460 nm, which was much larger than that of the Ni/Au film. It was also found that the specific contact resistances were 5.0×10-4 Ωcm2, 1.3×10-3 Ωcm2 and 7.2×10-4 Ωcm2 for the Ni/Au, Ni/ITO and 300°C-RTA Ni/ITO contacts on p-GaN, respectively. Nitride-based LEDs with these p-contact layers were also fabricated. It was found that the LED with the 300°C-RTA Ni/ITO p-contact has a reasonably small operation voltage (i.e., 3.29 V at 20 mA). The 20 mA output intensity of the LED with the 300°C-RTA Ni/ITO p-contact is also 65% larger than that of the LED with the Ni/Au p-contact.

-GaP Window Layer and a Thermally Annealed ITO Contact

The authors applied a simple, low-cost, mass-producible contact-transferred and mask-embedded lithography (CMEL) to texture p-GaP window layer for the fabrication of AlGaInP light-emitting diodes (LEDs) emitting at 612 nm. Under 20 mA current injection, it was found that forward voltages were 2.25, 2.39, 2.29, 2.39, 2.24, 2.21, and 2.25-V while the 20 mA output powers were 1.43, 1.42, 1.38, 1.35, 1.28, 1.22, and 1.16 mW for CMEL-400-nm LED, CMEL-600-nm LED, CMEL-800-nm LED, CMEL-1-μm LED, CMEL-2-μm LED, CMEL-3-μm, and the conventional LED without CMEL, respectively.

InGaN/GaN light-emitting diodes with rapidly thermal-annealed Ni/ITO p-contacts

The authors report the formation of air voids at GaN/cone-shaped pattern sapphire substrate interface by laser scribing and lateral etching with one-step growth. With 5 and 20 min lateral etching, it was found that pyramid-like air voids were formed with an average height of 0.98 and 1.9 μm, respectively, on top of each cone of the substrate. It was also found that we can enhance LED output power by 11.5% by etching the wafers for 20 min. It was also found that the simulated results agree well with the experimentally observed data.

AlGaInP LEDs Prepared by Contact-Transferred and Mask-Embedded Lithography

The authors report the formation of air-voids at the GaN/cone-shaped-patterned-sapphire-substrate interface by laser scribing and lateral etching with one-step growth. With 5- and 20-min lateral etching, it was found that pyramid-like air-void was formed with an average height of 0.98 and 1.9 μm, respectively, on top of each corn of the substrate. It was also found that we can enhance output power of GaN-based light-emitting diodes by 6.6% and 11.5%, respectively, by immersing the wafer in a mixture of H 3PO4 and H 2SO4 solution at 220 °C for 5 and 20 min, respectively.